1. Field of the Invention
The present invention relates to a wavetable synthesizer for usage in an electronic musical instrument. More specifically, the present invention relates to an apparatus and method of preprocessing sound samples for inclusion in a wavetable memory and usage in a wavetable synthesizer.
2. Description of the Related Art
A synthesizer is an electronic musical instrument which produces sound by generating an electrical waveform and controlling, in real-time, various parameters of sound including frequency, timbre, amplitude and duration. A sound is generated by one or more oscillators which produce a waveform of a desired shape.
Many types of synthesizers have been developed. One type of synthesizer is a wavetable synthesizer, which stores sound waveforms in a pulse code modulation (PCM) format into a memory and recreates the sounds by reading the stored sound waveforms from the memory and processing the waveforms for performance of defined sounds. The sound waveforms are typically large and a wavetable synthesizer generally supports the performance of many sounds including musical notes for a large number of musical instruments. Accordingly, one problem with wavetable synthesizers is the large amount of memory that is needed to store and produce a desired library of sounds. This problem is intensified by the continuing miniaturization of electronic devices which mandates smaller sizes while supporting evolutionary enhancements and improvement in performance.
Fortunately, the nature of sound waveforms aids the reduction in memory size since sound waveforms are highly repetitive. Various strategies have been developed which exploit this repetitiveness to save memory while accurately recreating sounds from recorded samples. These strategies generally involve identifying repetitive structures in the waveform, characterizing the identified structures, then eliminating the characterized structures from the stored waveform.
One technique for identifying and eliminating redundancy in a sound waveform is called looping in which, instead of retaining an entire waveform for a pitched sound, only the early portions of the sound are retained. Looping involves an analysis of a waveform to detect an interval at which the sample waveform becomes periodic or nearly periodic. Looping is effective since most pitched sounds become temporally redundant. Looping operations are sometimes combined with compression of the waveform and application of an artificial envelope. A physical characteristic of sound is that the sound decays in amplitude and frequency as time progresses. Looping of a decaying sound signal is facilitated by artificially flattening the amplitude of the sound signal.
High-quality audio reproduction using wavetable audio synthesis is only achieved in a system which includes a large amount of memory, typically more than one megabyte, and which commonly includes more than one integrated circuit chip. Such a high-quality wavetable synthesis system is cost-prohibitive in the fields of consumer electronics, consumer multimedia computer systems, game boxes, low-cost musical instruments and MIDI sound modules.
What is needed is a wavetable synthesizer having a substantially reduced memory size and a reduced cost while attaining an excellent audio fidelity. What is needed is a technique for reducing the memory size of a wavetable memory. What is needed is a technique of preprocessing sound waveform signals to reduce the amount of wavetable storage while retaining a quality sound upon playback.